Ring-shaped steel members such as gears having teeth on their outer circumferential surface are popularly used as parts in various types of mechanical apparatus. For example, a large number of gears including differential ring gears and counter gears are used in automatic transmissions in automobiles.
Generally, such ring-shaped steel members are required to have high-strength characteristics. An example of a method for imparting high-strength characteristics is an induction hardening process applied to the outer circumferential surfaces of the steel members.
The induction hardening process works extremely well as a method for enhancing the strength of the steel members, but also has a possibility of lowering the degree of precision in the dimensions. For example, after an induction hardening process is performed, the roundness of a ring-shaped part may be inferior to the roundness before the process is performed. If the part is of such a type that a loss of roundness does not lower its performance level, there is no problem. However, if the part is of such a type that a loss of roundness lowers its level of performance, it is necessary to add a correction process, after the induction hardening process is performed, in order to improve the roundness of the part.
For this reason, development of a method for preventing loss of roundness during an induction hardening process has been in demand. However, a satisfactory solution had not yet been found.
For example, Japanese Kokai 11-131133 discloses a jig for improving roundness of a circular-tube-shaped member after an induction hardening process has been performed on the inner circumferential surface of the circular-tube-shaped member. It is, however, not possible to use the disclosed jig to improve the roundness when an induction hardening process is performed on the outer circumferential surface of a part.